GIS data, flow monitoring, field measurements and a SWMM model were combined to quantify contribution from the different RDII sources. GIS layers and aerial maps were used to calculate runoff from the expected RDII source points in sanitary systems. RDII contribution from each source was calibrated using SWMM R-values which were calculated from historical storm events at which the surface and sub-surface were at saturation condition. Field data on individual RDII source types collected from two City of Columbus Study Areas were used in this analysis to improve the prediction of RDII contribution from each source. Splitting and quantifying RDII into its specific sources allowed for prioritizing an educated source control program in these basins. In addition, quantifying the surface runoff portion of the mitigated RDII sources will also be key to a successful integrated plan.